The US National Sleep Foundation takes a different approach. Its 2015 guidance on sleep time recommendations gives a core recommended range, a wider range of hours which may be appropriate for some individuals, and advice on what is considered too little or too much.8 (See table)

As such, its recommendations for 14-17-year olds is 8-10 hours' sleep, but no less than seven and no more than 11 hours.

For adults 18-64 years, the recommended range is 7-9 hours, with no less than six hours a night. However, while 18-25 years olds should not sleep more than 11 hours, those aged 26-64 years should not sleep more than 10 hours.

For people aged 65 upwards, the recommended sleep duration is 7-8 hours, but no less than five hours and no more than nine hours.